Seismic survey system for submerged areas



R. L. GALLAwAY SEISMIC SRVEY SYSTEM FOR SUBMERGED AREAS Oct. 23, 1951 .Filed Jan. 18, 1950 INVENToIL n.655 ALL/:WAY BY Patented Oct. 23, 1 951 2.572.255 sEIsMIc SURVEY SYSTEM Fon SUBMERGED AREAS Roy Lee Gallaway, Laurel, assignor to The Texas Company, New York, N. Y., a corporation of Delaware Application January 181950, Serial No. 139,248

4 Claims. l 'I'his invention relates to a seismic surveying system for submerged areas and more particularly to systems wherein a conductor cable having seismic detectors connected thereto is maintained at a predetermined level within the submerging medium, the conductor cable being in.

communication with receiving and recording instruments carried on a barge, vessel or other transporting means.

More specically, the invention contemplates a semi-floating conductor cable adapted to -be towed through the submerging medium having buoys spaced apart from each other and attached to the cable. A Weighting chain is attached to each buoy, one end of the chain being free to drag upon the submerged surface, the weight of the chains being such as to maintain the cable and its buoys submerged at a predetermined and substantially uniform elevation above the submerged surface. Seismic detectors supported in submerged carriers or buoys are connected at spaced intervals and by non-rigid or flexible means to the conductor cable, each detector being in electrical communication with the conductor.

In prior practice, the conductor cable with attached seismometers has been dragged over the submerged surface and this dragging is frequently injurious to the cableand may also involve wrapping of the individual seismometer leads around the conductor cable with attendant difculties as well as resultant adverse effects upon the quality of the recorded signals.

'I'he present invention with its employment of a semi-floating cable system possesses a number of advantages. In the rst place, the weight of the assembly is much less than that of the conventional cable and the pull required to move from one location to another is greatly reduced. A very important advantage is that the system permits use of multiple detectors as they are used on land, with resulting improvement of record quality. A still further advantage is that the detectors are not restrained under the extreme tension of the cable as they are in the conventional bottom type cable and, therefore, there is less chance of propagation of shock waves up and down the length of the cable with resultant interference in reilections.

For a further description of the invention, reference will now be made to the figures of the accompanying drawing.

Figure 1 indicates schematically, and in part, the semi-floating conductor cable with a plurality of detectors connected thereto.

Figure 2 indicates in more detail an individual cable buoy.

Figure 3 indicates in more .detail an individual detector buoy.

In Figure 1, the numeral I0 designates a :body of water while the numeral I I designates the surface of the submerged area. Numeral I2 designates a multiple detector marine cable equipped with the necessary electrical conductors. The

cable I2 leads to a winding drum or reel I3 l mounted on aboat I4.

of 1/2 linch iron chain. As indicated, the free end.

of the chain is adapted to drag on the submerged surface. A stabilizing chain I8 is connected to-/the tail end of the cable I2 so as to drag on the submerged surface and thus maintain suicient/tension on the cable to hold it in a straight line. It is contemplated that other stabilizing means may be employed. Thus when heavy currents of water exist, it may be advantageous to use a boat or other device.

As indicated in Figure l, a plurality of detectors or seismometer buoys 20a, 20h, 20c, 20d, etc., are connected by relatively short leads 2Ia, 2lb, 2Ic, 2Id, etc., to the semi-floating cable I2. Detectors 22a, 22h, 22e, 22d, etc., are rigidly attached to each of these buoys. 'Ihese detectors may be similar to that described in U. S. Patent No. 2,130,213, granted to Wolf, Cowles and Richardson, September 13, 1938, although other types of geophones may be used. f

Each detector is in electrical communication through a conductor in its respective lead with a conductor in, or associated with, the cable I2.

When a seismic or acoustic disturbance is created in the under-water surface of the earth, such as by the firing of a shot by a shot boat, not indicated in the drawing, the seismic wave travels outwardly through the earths substance. When the sound wave reaches a hard stratum in the earths substanceI a portion of the wave energy is reected with rays passing toeach detector. Each detector converts the wave energy received by it into electrical energy, as is well known and has been described, for example, in the foregoing patent.

The electrical energy produced in each detector is transmitted through theiectrical conductors in the aforementioned leads and in the cable I2 to nesting condition at au points and thus does not drag on the submerged surface. It is sdf vantageous to keep the detectors submerged a As indicated in Figure 2, the cable I2 may f comprise a steel cord or cable 25, V4 inch in diameter, extending through a tube 26, which in turnv extends axially'throug'h the cylindrical buoy I5.` l The buoy is locked in position on the steel cable 25 by means of cable clamps 21 and 28. The insulated electrical conductor'componentn of the compositefcable I2 extends along, the exterior of the buoy I5 andis rigidly fastened 'm'.claxnpedy to the sections of the steel cable 2i between buoys IIa, b, licglid, etc., by suitablez means weil known in the art. AThe buoy I5 may have `a volumetric capacity,v for example,.of about 500 s cubic inches and may be iilled with any suitable gas, such as air or nitrogen, etc.

vAs indicated in Figure 3, each detector buoy is connected by a steel cable 30, for example,

about '/u inch in diameter, to the composite cable I2. Each detector 22 is clamped or other- Vwise rigidly heldin supporting bracket' II attachedor welded to the lower or trailing end of its respective buoy 20. An insulated conductor 32 attached to the cable 30 provides a means of electrical communication between thel detector 22 and the electrical conductor component 29 of Figure 2. 'v

Advantageously, the bracket 3l is provided with a fin or vane 33.' While a single fin is shown, nevertheless several fins or vanes may be provided in any suitable geometrical arrangement such that the buoy 20 will assume an oblique position during movement of the cable I2 through the water, such as when being towed by the boat I4. 'I'he purpose of the fins or vanes is to prevent the detector buoys from diving and wrapping around the cable I2 during such tow- Each detector buoy 20 may have a volumetric capacity of about 280 cubic inches, for example. Each buoy 20 may be entirely filled with gas such as air or nitrogen. Advantageously, however, these buoys may be partially lled with a hydrocarbon oil, such as gas oil or kerosene. This oil servesl to adjust the buoyancy of the unit and also serves as a medium for suppressing natural resonance of the buoy. Thus in the absence of a fluid inside the detector. a substantial amount of high frequency noise may appear on the seismic records.

. In actual operation the buoyancy of the cable system -is adjusted so that all portions of the cable I2, to which detectors are connected, are maintained ata depth of about 1 1/2 feet, approximately, Aabove'the surface of the submerged area by means of the weighting chains. Thus with the cable system specifically described above, the chains I6 will be vabout 3 feet in length and weigh about 5 poundsper foot. The leads4 connecting the detectorv buoys may be approximately 3.5l feet in length so that the detector buoys are maintained approximately 5 feet above the surface of the submerged area but below the surface of the water. It will be realized, of course. that these distances can be varied, if desired, so long as the cable I2 is maintained in a semisubstantial distance below the surface of the water. In this way movement of the detectors as a result of sux'face movement of the .water is avoidedor reduced to a minimum.

It willbe understood that the Acable system is maintained at rest during shooting. After each shot has been recorded, the cable can be towed to the next location for recording of the next shot. Y

Obviously many modifications and variations of the invention, as hereinbefore set forth. may be made without departing from the spirit and scope thereof, but only such limitations should be imposed asv are indicated in the appended claims.

I claim: vl. A seismic surveying system for submerged area'sincluding a marine'cable adapted to bey ltowed'through the submerging medium, buoys .spaced apart from each other and attached to said cable, a weighting chain for each buoy aiatached by one end thereto with the free end ofy the chain adapted rto drag upon the surface of the submerged area, vand seismic detectors each supported ina submerged carrier, said carriers vbeing connected by non-rigid means to said cable and spaced apartv along the cable, each detector being in electrical communicationwith an electrical conductor in association with said cable, ,therelative weight of the said chains and buoy` ancy of the combined cable and buoys,` being such that the upward force of the buoyancy of the combined cableand buoys is greater than zero ibut less than the downward force of the entire weight of the chains, so that the whole assembly.

will stabilize at a level where the supporting of aportion of each chain by the ground surface leaves in balance the opposing upward force of the buoyancy and downward force of the unsupported portion of the chains. 2. A seismic surveying system for submerged vareas including` a marine cable adapted to be the chain adapted to drag upon the surface of thel submerged area, detector buoys non-rigidly. connected to said cable and spaced apart along the cable, and a seismic detector supported. by each detector buoy, each detector being in electrical communication with an electrical conductor in association with said cable. the relative weight of the said chains and b'uoyancy of the combinedcable and buoys being such that. the upward force of the buoyancy of the combined cable-and buoys'is greater than zero but less than the downward force of the entire weight of the chains. so that the whole assembly will stabilize at a level where the supporting of a portionof each chain by the ground surface leaves in balance the .opposing upward force of the buoyancy and downward force of the unsupported portion of the chains.

3. A seismic surveying system for submerged areas including a conductor cable adapted to be towed Athrough the submergingfmedium, buoys spaced apart from each other and attached to said cable, a weighting chain foreach buoy attached by one end thereto with the free end of the chain adaptedfto drag upon the submerged surface. detector buoys non-rigidly connected te i :aid cable and spaced apart along the cable and i seismic detector supported by each detector iuoy, each detector being in electrical communi- :ation with said conductor cable, the relative veight of -the said chains -and buoyancy of -he combined cable and buoys being such that he upward force of the buoyancy of the :ombined cable and buoys is greater than zero ut less than the downward force of the enire weight of the chains, so that the whole as- :embly will stabilize at a. level where the supiorting of a portion of each chain by the ground urface leaves in balance the opposing upward orce of the buoyancy and downward force of the msupported portionof the chains.

4. A seismic surveying system for submerged ireas wherein a conductor cable having a plu- 'ality of seismic detectors in electrical communi- :ation therewith is maintained submerged at a )redetermined level above the surface of the `ubmergcd area, the combination comprising a mnductor cable, a plurality of cable supporting uoys each spaced apart along the length of the L:able and attached thereto, a weighting chain '"'or each buoy having one end attached thereto lwhile the other end of the chain is free to drag ipon the submerged surface, and a plurality ot 8 detector buoys non-rigidly connected to said cable and spaced apart along the cable, the relative weight of the said chains and buoyancy of the combined cable and buoys being such that the upward force of the buoyancy of the combined cable and buoys is greater than zero but less than the downward force of the entire weight of the chains, so that the whole assembly will stabilize at a level where the supporting of a portion of each chain by the ground surface leaves in balance the opposing upward force of the buoyancy and downward force of the unsupported portion of the chains.

ROY LEE GALLAWAY.

REFERENCES CITED The following references are of record in the file oi' this patent:

UNITED STATES PATENTS Number Name Date 2,203,894 Cooke June 1l, 1940 2,241,428 Silverman May 13, 1941 FOREIGN PATENTS Number Country Date 3,654 Great Britain of 1884 

